Since the advent of carbon nanotubes (CNT) [Iijima 1991; Oberlin 1976] engineering and controlled synthesis of these materials have been thoroughly investigated. Doping multi-walled carbon nanotubes (MWCNT) and single-wall carbon nanotubes (SWCNT) with elements such as nitrogen and boron has also been used for altering their electronic properties for specific applications. [Stephan 1994; Suenaga 1997; Terrones 1996; Sen 1998; Nath 2000; Terrones 1999; Yudasaka 1997; Suenaga 2000; Wang 2002; Kurt 2001; Ma 1999; Redlich 1996; Sen 1997]. Boron doping of fullerenes by substitution was theoretically discussed [Han 2000]; and previous research demonstrated that boron-doped multi-walled carbon nanotubes (CB×MWNTs) can be formed via chemical vapor deposition (CVD) using BF3/MeOH as the boron source, a complex boron supported catalyst Fe/Ca (BO3)2/CaCO3, and acetylene as the carbon source. [Mondal 2007] It was also found that “sea-cucumber” morphologies could be formed when carrying out the spray-pyrolysis of a ferrocene-xylene-triethylborane (TEB) mixture. [Lozano-Castello 2004]. CB×MWNTs have been synthesized using 1M triethylborane in hexanes solution mixed with toluene and compared to CN×MWNT's. [Koós 2010]. Other methods for synthesizing double-walled and single-walled CB×NT's have also been reported [Lyu 2011; Goldberg 1999; Maultzsch 2002; Redlich 1996; McGuire 2005].
It has also been found that dopant atoms such as nitrogen or sulfur can induce dramatic tubule morphology changes in CNTs, including covalent multi-junctions [Sumpter 2007; Romo-Herrera 2008; Romo-Herrera 2009; Sumpter 2009], however these morphologies were not utilized to create 3D macro-scale architectures.
Theoretical and experimental studies on the electronic structure of both semiconducting and metallic CB×NT's have shown a strong acceptor state due to the presence of boron and a lowering of the Fermi level. [Yi 1993; Carroll 1998]. Theoretical studies have predicted that significant structural reorganization generates stable bends in CNTs due to presence of pentagon and heptagon defects [Dunlap 1992] that could accommodate foreign atoms besides carbon within the sp2 graphitic lattice [Sumpter 2009]. In addition, it was found that boron doping acts as a “surfactant” during growth to significantly increases the aspect ratio of nanotubes by preventing tube closure—allowing longer tube lengths to be synthesized (˜5-100 μm) and favoring the zigzag (or near zigzag) chirality [Blasé 1999]. It was later found that dopant atoms can also induce dramatic tubule morphology changes in CNT's. [Lee 2002; Sumpter 2007].
CB×MWNTs could be synthesized by chemical vapor deposition (CVD) using multiple hydrocarbons and boron sources [Mondal 2007; Lozano-Castello 2004; Koós 2010; Lyu 2011], but none of these works yielded macroscale 3D solid structures, or were able to confirm the distinct tubular morphologies induced by boron.
Theoretical and experimental research had demonstrated that boron interstitial atoms located between double-walled CNTs act as atomic “fusers” or “welders” under high temperature annealing (1400-1600° C.) [Endo 2005], thus establishing covalent tube interconnections, but neither did this work produce macroscale solids. 3D solids of straight entangled non-doped CNTs were recently reported by others to create compressible sponges [Gui II 2010] and temperature-invariant viscoelastic solids [Xu 2010]. However neither of these works show promise towards any degree of covalent bonding established between CNTs; nor do they possess dramatic defect sites within the CNT network.
There is a need for a scalable synthesis process for building macroscale three-dimensional structures from one-dimensional (1D) CNT building blocks. As used herein, a “macroscale three-dimensional structure” (or “macroscale 3D structure”) is a material that is at least 1 cm in three orthogonal directions. The macroscale 3D structure composed of CNTs can be obtained by: (1) randomly aligned, isotropic ensemble of entangled nanotubes without requiring nanotube-nanotube junctions; (2) randomly aligned isotropic ensemble or an ordered array nanotube structure containing two-dimensional nanotube junctions; (3) randomly aligned isotropic ensemble or an ordered array nanotube structure containing three-dimensional nanotube junctions; (4) a structure composed of any combination of (1), (2) and (3). As further used herein, a “junction” is considered to be any form of covalent bonding between the nanotubes at any (all) angle(s).
As further used herein, a “two-dimensional nanotube junction” (which is also referred to as a “two-dimensional nanotube” or “2D CNT”) is a nanotube that is a least 100 nm in two perpendicular directions (or in the same 2D plane having any angle), while, in the direction orthogonal to both perpendicular directions (2D plane), the nanotube is generally less than 100 nm. For example, a nanotube in the shape of a cross (or an “X”) that has a length of more than 100 nm along the vertical axis and a width of more than 100 nm along the vertical axis, but has a depth less than 100 nm, would be a two-dimensional nanotube junction. When a two-dimensional nanotube junction is on the macroscale, this means the carbon nanotube is at least 1 cm in two perpendicular directions, and can be referred to as a “macroscale two-dimensional nanotube junction” (or a “macroscale two-dimensional nanotube.”)
Coordinately, a “three-dimensional nanotube junction” (which is also referred to as a “three-dimensional nanotube” or “3D CNT”) is a nanotube that is a least 100 nm in three orthogonal directions. When a three-dimensional nanotube junction is on the macroscale, this means the carbon nanotube is at least 1 cm in three orthogonal directions, and can be referred to as a “macroscale two-dimensional nanotube junction” (or a “macroscale three-dimensional nanotube.”)
In the present invention, the heteroatom-doped carbon nanotube material (such as CB×NT material) synthesized by the AACVD process of the present invention is a macroscale 3D structure (or macroscale 3D material). The heteroatom-doped carbon nanotubes in this macroscale 3D structure are not necessarily macroscale three-dimensional nanotubes. Generally, in the absence of a post-synthesis process (such as welding) the heteroatom-doped carbon nanotubes in the macroscale 3D structure are two-dimensional nanotubes, and, in some instances, macroscale two-dimensional nanotubes, or purely entangled randomly aligned one-dimensional nanotubes.
After the post-synthesis process (such as welding), the nanotubes in the macroscale 3D structure can themselves be three-dimensional nanotubes, and, generally, macroscale three-dimensional nanotubes. In this way the structure can become virtually monolithic solids composed of macroscale three-dimensional nanotubes.